Embodiments of the inventive concept described herein relate to a technique for removing residual interference caused by a non-orthogonal filter in a quadrature amplitude modulation-filter bank multi-carrier (QAM-FBMC) system.
Cyclic Prefix-Orthogonal Frequency Division Multiplexing (CP-OFDM) is a technique that may be easily applied to very many applications such as LTE, Wi-Fi and the like and may easily employ complex signal processing such as a multi-input multi-output (MIMO) technique.
However, the CP-OFDM has a high out-of-band emission property in a frequency domain due to a rectangular waveform symbol, so that the frequency limitation property is bad. Thus, due to the bad frequency limitation property, in case of LTE, a quite number of guard bands are used so that the bandwidth efficiency is deteriorated.
In addition, since CP is used to prevent an orthogonality loss due to the inter symbol interference (ISI) of an OFDM system, a loss is forced even on a time axis. The CP-OFDM property does not meet the flexibility required in next generation wireless communication environment. Specifically, the OFDM system of which the time and frequency localization properties are insufficient cannot avoid a loss in orthogonality in an asynchronous environment such as narrowband-Internet of things (NB-IoT). Thus, there is required a new technique which can meet both the time and frequency localization properties and orthogonality.
As one technique proposed to overcome the defects of CP-OFDM, there has existed OQAM-FBMC. The FBMC is a technique that improves the frequency limitation property by applying filters to sub-carriers, respectively and enables a flexible waveform design to meet various scenarios. Specifically, the OQAM-FBMC achieves orthogonality on a real axis by using the OQAM modulation so that the OQAM-FBMC is almost similar to the OFDM in terms of performance. When a symbol experiences a real complex channel and a MIMO scheme is applied, the residual interference of an imaginary part is introduced so that the orthogonality on a real axis is damaged. A scheme of solving the problems caused due to the residual interference is required in the OQAM-FBMC. Thus, in recent years, there has been proposed a QAM-FBMC scheme for solving the problems. There has been proposed a QAM-FBMC filter design to sufficiently meet generalized Nyquist condition through the global optimization of a filter factor. Since there is a trade-off between the filter orthogonality and the frequency limitation property, it is impossible to meet the filter orthogonality and the frequency reduction rate in the filter design. Although a certain degree of orthogonality and a high frequency limitation property are guaranteed by applying a specific filter to the QAM-FBMC system, since the filter does not have perfect orthogonality, so that the performance is limited due to the residual interference component.
Therefore, a technique of effectively removing the residual interference of a non-orthogonal filter in QAM-FBMC is required.